Ultra-compact cooling systems based on phase change material heat reservoirs

ABSTRACT

A cooling system includes a first stage heat reservoir arranged to absorb heat from a heat source. Heat is transferred from the first stage heat reservoir to a second stage heat reservoir. The first stage heat reservoir includes a material with a heat capacity lower than that of the second stage heat reservoir but with a thermal conductivity higher than that of the second stage heat reservoir. Heat transfer structure increases heat transfer rate from the first stage heat reservoir to the second stage heat reservoir.

FIELD OF THE INVENTION

The present invention relates generally to cooling systems, such as forelectronics and the like, and particularly to a cooling system thatemploys phase change materials.

BACKGROUND OF THE INVENTION

Cooling of power electronics components and diode-lasers hastraditionally been performed using systems that transfer the heat to theenvironment (e.g., thermo-electric coolers, chillers, heat pipes,air-blown heat-exchangers, radiators). Many platforms (missiles, pods,satellites) do not readily allow for the elimination of heat generatedwithin the system because of size, weight, electrical power, operatingscenario limitations or other factors. For such applications, theconcept of heat reservoirs to store the heat close to its source untilcompletion of a mission can impart significant advantages in terms ofsimplicity, weight, and low power requirements. While heat reservoirscan operate on the basis of raising the temperature of a material withmoderate heat capacity, the use of phase change materials (PCMs) hassignificant benefit in terms of much higher specific heat capacity andmuch lower increases in temperature.

However, a disadvantage of PCMs is their low thermal conductivitycompared to standard heat sink metals.

SUMMARY OF THE INVENTION

The present invention seeks to provide a novel cooling system thatemploys phase change materials, as is described more in detailhereinbelow.

The present invention overcomes the low thermal conductivity of PCMsthrough the use of multi-stage heat reservoirs: a first stage isoptimized to contain heat generated during a single heat pulse of finiteduration, and a second stage uses a high-capacity heat-reservoiroptimized to absorb heat between pulses. The invention may also useadvanced heat transfer structures, such as densely packed, thin,diamond-coated copper fins extending throughout the PCM.

The operational challenge in adopting PCM based passive cooling systemsis adapting heat generation to a periodically pulsed format and adaptingthe source to operate effectively despite a temperature that variesbetween the phase change temperature (equal to the initial systemtemperature) and the maximum allowable source temperature. Designsimulations show that duty factors of many tens of percent can beachieved and that maximum temperatures can be maintained to belowtypical laser diode maximum temperatures.

Power electronics components and diode lasers generate heat fromconcentrated areas. Heat densities can reach 1 kW/cm². This heat loadmust be removed from the area of the heat source and then dealt with.Traditional high capacity cooling systems remove the heat from thesystem and dump it into the environment. Examples are Freon basedchillers, thermo-electric coolers, and forced convectionheat-exchangers. Some lower capacity cooling systems utilize heat pipesto transport the heat from the source to the point where it is expelledto the environment.

Another approach would be to store the heat within the system. This hasbeen done in the past by incorporating large amounts of heat spreadermetal in the mechanical design. In systems containing laser diodes thematerial most often used is aluminum [C_(p)=0.9 J/(gm·K), K=165 W/(m·K)]since it is used anyway for the base to which all of the optics aremounted. In power supplies, copper [C_(p)=0.38 J/(gm·K), K=385 W/(m·K)]might be the material of choice because of its high electricalconductivity. If heat transfer rate were not a factor, then considerableweight could be saved by switching to some higher heat capacitymaterial. Water [C_(p)=4.2 J/(gm·K), K=0.58 W/(m K)] is but one example.If the heat reservoir is allowed to increase in temperature by 10° C. inorder to store heat energy, then 100 gm of water can hold 4.2 KJ. Phasechange materials (PCMs) are much more effective absorbers of heat.Hexadecane, for example, melts at 18° C. (C_(p)=2.1 J/(gm·K),C_(latent)=230 J/gm, K=0.15 W/(m·K) and 100 gm will absorb 23 KJ ofthermal energy at the melting point and an additional 2.1 KJ if thetemperature shifts by 1° C. around the melting point.

Thermal conductivity strongly affects the peak temperature duringheating. Thus, placing the heat source on a simple water or PCM cellwill not work. The present invention proposes some solutions, such as:a) use a material with lower heat capacity but reasonably high thermalconductivity, b) design a heat reservoir containing fins or other heattransfer structure to speed up the heat flow, or c) use a two-stage heatreservoir where the first stage has high power handling capacity but lowintegral energy storage capacity and a second stage optimized for highenergy storage capacity. One embodiment of the present inventioninvolves the first option in designing the first stage, and the secondand third options in designing the second stage.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with thedrawings in which:

FIG. 1 is a simplified illustration of a cooling system, in accordancewith a non-limiting embodiment of the invention;

FIG. 2 is a simplified illustration of time profiles for the heatingpulse and for the temperature of the heat source under different coolingscenarios, for the cooling system of FIG. 1;

FIGS. 3A and 3B are illustrations of long and short-term temperaturevariations, respectively, in a single-stage aluminum heat reservoir, inaccordance with a non-limiting embodiment of the invention;

FIG. 4 is a simplified illustration of a two stage heat reservoir withthe first stage as in FIGS. 3A-3B but with an added water-based 2^(nd)stage, in accordance with a non-limiting embodiment of the invention;

FIGS. 5A and 5B are simplified illustrations of using two different PCMmaterials: hexadecane (FIG. 5A) and gallium (FIG. 5B), in accordancewith another non-limiting embodiment of the invention; and

FIG. 6 is a simplified illustration of two-stage heat-reservoircool-down times as a function of the number of fin cooling surfaces, inaccordance with a non-limiting embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is now made to FIG. 1, which illustrates a cooling system, inaccordance with a non-limiting embodiment of the invention.

A heat source (such as an electronics or laser component/system) givesoff heat, such as a heating pulse. A first stage heat reservoir absorbsthe heat from heat source during the heating pulse. The heat is thentransferred to a second stage heat reservoir. The dual-stage heatreservoir cooling system is located within a thermally isolated system.

FIG. 2 illustrates time profiles for the heating pulse and for thetemperature of the heat source under different cooling scenarios.

The upper part of the graph shows a burst of N heating pulses. The lowerpart of the graph shows the temperature increase based on “sensible”heat reservoirs in which thermal energy results in a temperatureincrease, or based on a 1st stage sensible heat reservoir and a 2ndstage PCM heat reservoir. (Sensible heat transfer causes change oftemperature of the system while the given state [solid, liquid or gas]remains unchanged.) T0 is the initial temperature, T1 is the “spike”temperature at the end of the heating pulse, T_(limit) is the maximumallowable temperature, and T_(PC) is the phase change temperature.

Before making a judgement on the superiority of PCM based heatreservoirs, simulation results should be compared. Three-dimensionaltime-dependent simulations were performed using the finite elementprogram ANSYS. Samples are shown in FIGS. 3A-5B.

FIGS. 3A and 3B illustrate long and short-term temperature variations,respectively, in a single-stage Al heat reservoir. Equilibrium isreached within 25 sec after the cessation of the 15 sec long heatingpulse. The single stage Al heat reservoir is optimally sized to minimizeT1 at minimum weight.

FIG. 4 shows a two stage heat reservoir with the first stage as in FIG.3 but with an added water-based 2^(nd) stage. FIG. 4 shows T_(max) andT_(min) vs. time for 27 mm Al base plus 79 mm water reservoir with 7fins. Heating pulses are every 1800 sec and T0=20° C. While the watersubstantially reduced T2, the equilibrium time was increased (despitethe addition of seven heat transfer fins).

FIGS. 5A and 5B show two different PCM materials: hexadecane (FIG. 5A)and gallium (FIG. 5B). The heat reservoirs were sized to hold the sameamount of energy (7.6 KJ in four pulses). Note the difference in timescales. FIGS. 5A and 5B show T_(max) and T_(min) vs. time for 27 mm Albase plus PCM reservoirs with 7 fins. T_(PC) for hexadecane and galliumare 16 and 30° C. respectively. The minimum temperature rises during thephase transition zone because of the latent heat model were patched intoANSYS. Other melting points can be found for both classes of materials(paraffins and low melting point metals). Hexadecane is even slower thanwater. Gallium approaches the response time of aluminum.

Table 1 summarizes the weights of two-stage heat reservoirs sized tostore 7.6 KJ of heat. The Al—Al heat reservoir includes a block ofaluminum placed after the 27 mm block optimal for use with 15 secpulses. The weight advantage of using PCMs is clear. Their maindraw-back is slow equilibration time.

Type of two-stage heat reservoir Weight-gm Sensible-heat Al—Al 8560Al-Water 960 PCM Al- 120 hexadecane Al-metal 390 alloy

The present invention surprisingly can reduce the heat transfer times tothe PCM. The problem is the low thermal conductivity of the PCM comparedto that of the 1^(st) stage. Heat transfer can be increased by reducingthe distance that the heat must travel through the PCM, and byincreasing the surface area of the heat transport structure. One way ofachieving this is to increase the number of fins. This was simulated byreducing the fin thickness as the number of fins increased. This keptconstant the amount of PCM in the reservoir. In order to insure thatlongitudinal heat flow does not limit heat transfer to the PCM, theinventors simulated the use of a diamond-copper-diamond sandwich withK=800 W/(m·K) of the type developed at Civan, Israel, for laser-diodesub-mount/heat-spreader applications. Results for hexadecane are shownin FIG. 6. A dramatic reduction is possible. Equilibrium time wasreduced by 35×. At these heat transfer rates, one starts to be limitedby the transport through the first stage. The cooling system can beoperated in the medium to high duty-factor regime, especially ifcomplete thermal equilibrium is not required. Even faster equilibriumtimes were simulated with gallium and the high thermal conductivityfins.

FIG. 6 illustrates two-stage heat-reservoir cool-down times as afunction of the number of fin cooling surfaces. (The outer two fins haveonly one cooling surface.). Cool-down times are measured from the startof the heating pulse. The solid line is consistent with complete thermalequilibrium. The dotted lines are consistent with T2−T_(PCM)=1 or 2° C.The cooling system can be operated in this mode. At 65 cooling surfaces,response time starts to be limited by heat transfer time through the1^(st) stage reservoir.

Fins can be produced with an overall fin thickness of approximately 150μm for the 100 surfaces case. The copper foil may be 50 μm and thenano-diamond coatings on both sides may be 50 μm each.

In conclusion, extremely light-weight cooling systems can be developedon the basis of multi-stage heat-reservoirs that contain phase changematerials as the final storage medium. Breakthrough enhancement inrecovery time comes about when applying heat fins (such asdiamond-copper-diamond fins) to the PCM based reservoirs.

What is claimed is:
 1. A cooling system comprising: a first stage heatreservoir arranged to absorb heat from a heat source; a second stageheat reservoir to which heat is transferred from said first stage heatreservoir, said first stage heat reservoir comprising a material with aheat capacity lower than that of said second stage heat reservoir butwith a thermal conductivity higher than that of said second stage heatreservoir; and heat transfer structure that increases heat transfer ratefrom said first stage heat reservoir to said second stage heatreservoir.
 2. The cooling system according to claim 1, wherein saidfirst stage heat reservoir comprises a sensible heat reservoir and saidsecond stage heat reservoir comprises a phase change material (PCM) heatreservoir.
 3. The cooling system according to claim 1, wherein said heattransfer structure comprises cooling fins.
 4. The cooling systemaccording to claim 3, wherein said cooling fins comprise a diamond andcopper sandwich structure.
 5. The cooling system according to claim 1,wherein said PCM comprises gallium.
 6. The cooling system according toclaim 1, wherein said PCM comprises hexadecane.